The invention relates to telemetry communication, and in particular to uplinks from an implanted medical device.
In the field of programmable implantable medical devices, it has become common to provide an interactive transceiver system that transmits uplinks to and receives downlinks from an external medical device. Downlinks may include, for example, programming of operating functions, modes and parameters. Uplinks may include, for example, physiologic data related to the condition of the patient having the implantable device, as well as data pertaining to the programmed operating functions, modes and parameters of the device.
Implantable medical devices include cardiac pacemakers, cardiac and other physiologic monitors, implantable drug dispensers, nerve, muscle, and brain stimulators of various types, cochlear implants, blood pumps, cardiomyostimulators, and tachyarrhythmia-control devices such as implantable cardioverter/defibrillators (ICD""s) for delivery of staged therapies to the ventricles and/or the atria. Each of these devices may include a transceiver system, also called a telemetry system.
The telemetry system in the implantable device typically communicates with the external device using radiated electromagnetic signals. For example, the implanted device and the external device may communicate using radio frequencies.
In some implanted devices, sources of noise are generated internally that interfere with telemetry. Some implanted devices include an inductive element, for example, that emits electromagnetic noise when activated. In devices that deliver electric therapy, such as ICD""s that deliver defibrillation pulses, a storage element such as a capacitor is charged to a high voltage, and an inductive element is employed in the charging.
When a patient with an ICD experiences a condition that may require defibrillation, the ICD stores energy in the storage element for delivery to the patient. At this time, data concerning the condition of the patient may be of interest to the patient""s physician. Accordingly, the physician may be concerned with the data provided by telemetry, but telemetry may be subject to electrical interference from the energy storage circuitry.
The invention is directed to techniques for reducing the interference to telemetry from sources of controllable noise in an implantable medical device. The invention reduces the interference by suspending the noise source during telemetry. The invention is described in the context of an implanted system that includes a defibrillator system and a telemetry system, but the invention is not limited to that context. The invention may be applied to a variety of implanted medical devices that employ telemetry and have noise sources that can be suspended during telemetry.
In an implanted system with a defibrillator system, a charging circuit stores energy for defibrillation in a storage element such as a capacitor. Energy storage involves delivery of charging current to a capacitor. Unfortunately, the charging circuit can be a source of electromagnetic noise that interferes with telemetry. The invention reduces the electrical interference caused by the charging circuit by temporarily suspending charging during telemetry.
Simply switching off the charging circuit when telemetry takes place, however, may produce an undesirable effect. In particular, the charging circuit may generate a noise spike if switched off abruptly, and this noise spike may result in the detection of false cardiac signals by monitoring electrodes associated with the device. The false signals, in turn, may adversely affect the ability of the implanted device to accurately detect true cardiac signals.
Accordingly, the invention provides for suspending the operation of the charging circuit gradually rather than abruptly. The charging circuit includes a clock that generates a control signal having a period and a duty cycle. The invention provides for suspending charging by decreasing the duty cycle while keeping the period unchanged. The gradual decrease of the duty cycle reduces the risk of detecting false cardiac signals. The clock reduces the duty cycle to a predetermined level at which the noise no longer interferes with telemetry. In a typical application, the predetermined duty cycle level is zero. Once the charging cycle has been suspended, telemetry may take place without substantial risk of electrical interference due to noise generated by the charging circuit.
In one embodiment, the invention provides a method comprising suspending storing energy in an energy storage device in an implanted defibrillator and initiating a telemetry communication upon suspending the energy storage. Energy storage may be suspended by reducing a duty cycle of a clock that controls delivery of energy to the energy storage device.
In another embodiment, the invention presents a method comprising reducing a duty cycle of a clock and, when the duty cycle has been reduced to a predetermined level, transmitting a radio frequency message. The method may comprise reducing a duty cycle of a clock that controls storage of energy in an energy storage device in an implanted defibrillator.
In a further embodiment, the invention presents a medical device comprising a transceiver, a charging circuit and a controller that disables the charging circuit prior to initiating a telemetry communication via the transceiver. The charging circuit may include a switch to control a supply of energy in response to a duty cycle of a charge clock, and the controller may disable the charging circuit by reducing the duty cycle of the charge clock.
The invention improves telemetry communication by reducing electromagnetic noise that interferes with the telemetry, thereby enhancing the signal-to-noise ratio. The invention has a further advantage of reducing noise without introducing false signals.
The above summary of the invention is not intended to describe every embodiment of the invention. The details of one or more embodiments of the invention are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the invention will be apparent from the description and drawings, and from the claims.